The initiation of protein synthesis in eukaryotic cells requires the participation of ribosomal subunits, a specific initiator aminoacyl-tRNA molecule, messenger RNA, and specific protein initiation factors that bring about the interaction between ribonucleoprotein particular, protein factors, and various nucleic acids. Several activities that carry out initiation reactions have been identified among the non-ribosomal proteins that are associated with ribosomal subunits. For example, in addition to eIF-2, (the template-independent binding factor which makes a ternary complex with Met-tRNAf and GTP), another binding factor (IF-1) is present which is also specific for Met-tRNAf but is GTP-independent and template-dependent. Also, in addition to the high molecular weight (about 0.5 x 10 to the 6th power daltons) eIF-3 ribosome dissociation factor, composed of 8-10 polypeptides, evidence for a low molecular weight dissociation factor (IF-3L) has been obtained. The mechanism of action of these factors is under investigation. The IF-1 molecule has been isotopically labeled by reductive alkylation. The binding of labeled IF-1 to various ribosomal particles, and the requirements for its interaction with 40S ribosomal subunits and Met-tRNAf will be examined. The fate of 40S-bound IF-1 when 60S subunits join to make an 80S initiation complex will also be looked at. The manner in which the light form of IF-3 behaves in the ribosome-dissociation or the subunit anti-association reactions, and the nature of the complexes that are formed between ribosomal subunits and the protein will be investigated. Attempts will be made to purify IF-3L and compare with IF-3H, to determine whether they contain any polypeptides in common.